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Application of a thermodynamic model to predict the mineral stability of titaniferous smectites from soils on volcanic rocks

Published online by Cambridge University Press:  09 July 2018

J. L. Martin de Vidales ,
J. Casas  and
J. Sanz
J. L. Martin de Vidales
Affiliation:
Departamento de Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco28049-Madrid
J. Casas
Affiliation:
Instituto de Edafologia y Biologia Vegetal, Consejo Superior de Investigaciones Científicas, Serrano, 115, 28006-Madrid, España
J. Sanz
Affiliation:
Departamento de Geología y Geoquímica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco28049-Madrid
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Abstract

Weathering processes in the Piedrabuena volcano, Ciudad Real province, Central Spain, have produced a smectitic phyllosilicate in four profiles on porphyritic-holocrystalline basalts consisting mainly of titaniferous augites, olivine and pigeonite. Chemical analysis of the purified smectites gave TiO2 contents of 1·89–3·77%. Apart from one profile, the smectites showed high tetrahedral Al-for-Si substitution, with the lattice charge arising from this source exceeding that from octahedral substitution, and thus could be termed beidellites. Octahedral Ti contents ranged from 0·21 to 0·41 per 24 O. A thermodynamic model was used to estimate the ΔG° of formation of these titaniferous smectites. Values ranged between −2503·30 and −2543·26 Kcal/mol. Suitable equilibria were established by calculating their ΔG° of reaction and log Ksp and, from these, the stability diagrams pH - 1/2pMg2+ against log H4SiO4 were derived. Correlations found were: (i) strong and positive between AlVI-X+ (0·91), AlIV-FeVI (0·85), AlVI-FeVI (0·85) and AlVI-MgVI (0·76); (ii) strong and negative between AlIV-octahedral charge; (iii) solubility product (log Ksp) positively correlated with X+ (0·73), AlIV (0·89), FeVI (0·67) and MgVI (0·84). Smectites showing higher tetrahedral charges were the least stable. Conversely, the stabilities of titaniferous smectites were favoured by high values of pH - 1/2pMg2+ and/or activities in soil-matrix solution. When the activities of H4SiO4 decreased, the montmorillonitic smectites became less stable than those showing beidellitic characteristics. Variations in composition and stability of the titaniferous smectites were not correlated with soil depth.

Type
Research Article
Information
Clay Minerals , Volume 21 , Issue 3 , September 1986 , pp. 389 - 400
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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